Transmitters and Local Controllers 3twanclik.free.fr/electricity/IEPOPDF/1081ch3_1.pdf457 Transmitters and Local Controllers 3 3.1 CONTROLLERS—PNEUMATIC 460 History and Development

457

Transmitters and LocalControllers

3

3.1 CONTROLLERS—PNEUMATIC 460

History and Development 461Operating Principles 461

Booster Circuit 462Proportional Response 462Reset or Integral Response 463Derivative Response 464

Miniature Controller Designs 464Derivative Relay 465Miniature Control Stations 466High-Density Stations 470

Large-Case Designs 473Receiver Controllers 473Direct-Connected Controllers 473

Field-Mounted Controllers 474Direct-Connected 474Receiver-Type 474Pneumatic with Electronic Detectors 474

Special Pneumatic Controllers 475Connections for Digital Highways 475Special Controls 475

Bibliography 477

3.2CONTROLLERS—ELECTRONIC 478

Introduction 479Analog Electronic Controllers 480

Analog On/Off Switch 480Analog PID Controller 480

Digital Electronic Controllers 483Performance 484

Communication Capabilities 484Microprocessor-Based Controllers 484Operation 484Hardware 484Software 486

Conclusions 487References 487Bibliography 487

3.3 CONVERTERS AND DAMPENERS 488

Introduction 488Pneumatic Converters and Dampeners 488

Pulsation Dampeners 489Digital Converters 489Analog Converters 489

Pneumatic-to-Electronic Converters 489Current-to-Air Converters 489Millivolt-to-Current Converters 491Voltage-to-Current Converters 491Current-to-Current Converters 491Resistance-to-Current Converters 492

Electronic Noise Rejection 492Bibliography 492

3.4RELAYS FOR COMPUTING ANDPROGRAMMERS 493

Introduction 495Mathematical Functions 495Pneumatic Relays 495

Multiplying and Dividing Relays 495

© 2006 by Béla Lipták

458 Transmitters and Local Controllers

Adding, Subtracting, and Inverting Relays 496Differentiating Relay 497Scaling and Proportioning Relays 498Integrating Relay 498Square Root Extractor Relay 499High- and Low-Pressure Selector

and Limiter 499Electronic Computing Elements 500

Multiplying and Dividing 500Adding, Subtracting, and Inverting 500Differentiating 501Integrating 501Scaling and Proportioning 501Square Root Extracting 501High- and Low-Voltage Selector

and Limiter 502Timing Elements 502

Analog Timing Operations 502Digital Timing Operations 503

Programmers 503Step Programmers 503Profile Programmers 503

Bibliography 505

3.5TELEMETERING SYSTEMS 507

Introduction 508Basic Telemetry Concepts 508

Classical Configuration 508Recent Trends 509

Operating Principles and Types 509Base-Band Telemetry 510Multiple-Channel Telemetry 510

Telemetry Protocols, Standards,and Networks 513

Protocols and Standards 513Networks 515Telemetry Equipment and Hardware 515Sensor Networks 516

Telemetry Applications 516Medical and Life Science Applications 516Industrial Telemetry 518Space Telemetry 518Other Applications 519

Bibliography 519

3.6TRANSMITTERS—ELECTRONIC 520

Transmitter Configurations 521Two-Wire Loops 522Three- and Four-Wire Loops 522

Measured Variables 523Temperature 523Pressure 525

Flow 527Level 528Motion 529

Important Features 530Inaccuracy 530Zero Suppression and Elevation 530Turndown 531External Calibration 531Intrinsic Safety 531

Digital Communication 531Transmitter Selection 532

Intelligent Transmitters 532Desirable Features Checklist 532

References 533Bibliography 533

3.7TRANSMITTERS—FIBER-OPTIC TRANSMISSION 535

Introduction 536Fiber Optics Basics 536System Components 537

Fiber Sizes and Properties 538Single- and Multi-Mode Fibers 538Pulse Width Distortion 539Fiber Selection 539Fiber-Optic Cable 540Fiber-Optic Connectors 541

Receivers and Transmitters 542Modems 542Modem Selection Criteria 543

Installation and Testing 543Cabinets 543Splicing 543Pull Strength and Bending Radius 544Testing 544

Conclusions 545References 545Bibliography 545

3.8TRANSMITTERS—PNEUMATIC 547

Introduction 548History 548General Features 548

Signal Ranges 548Baffle-Nozzle Error Detector 548

Force-Balance Devices 549One-to-One Repeaters 549Membrane d/p Cells 550Pressure Transmitter 551

Motion-Balance Transmitters 552Transmitters Grouped by Measured Variable 552

Differential Pressure Transmitter 552Square Root–Extracting d/p Transmitter 553


Contents of Chapter 3 459

Variable Area Flow Transmitter 553Filled Bulb Temperature Transmitter 554Buoyancy Transmitter (Level or Density) 554Force Transmitter 555Motion Transmitter 556Speed Transmitter 557

Transmission Lag 557Bibliography 558

3.9TRANSMITTERS: SELF-CHECKINGAND SELF-VALIDATING 559

Introduction 559Levels of Diagnostic Information 559

How Diagnostics Are Performed 559Diagnostics Transmission 561

Analog Transmitters 561Microprocessor-Based Transmitters 561

Diagnostic Information Displays 563Portable and Handheld Displays 563

Acting on the Diagnostic Data 565

Failsafe and Alarm Actions 565Reference 566

3.10TRANSMITTERS: SMART, MULTIVARIABLE,AND FIELDBUS 567

Introduction 567Operation and Performance 567

Digital Sensors 568Sensor Compensation and Characterization 568Multivariable and Inferential Sensors 569HART Communication 570Fieldbus Transmitters 570

Benefits of Advanced Transmitters 570Savings 570Installation and Commissioning 571Parameterization 571Integration into DCS and PLC Systems 572Building Control Loops 573

Reference 574


460

3.1 Controllers—Pneumatic

C. L. MAMZIC (1970, 1985, 1995) P. M. B. SILVA GIRÃO (2005)

Types: Receiver controllers: Indicating, recording, miniature, high-density miniature, andlarge case

Direct-connected Blind, indicating, or recording, in large or medium case for field or panel mounting.controllers:

Application: Receiver controllers: Control of any variable that can be measured and translated intoan air pressure by a pneumatic transmitter; includes automatic and manual controland set point adjustment.

Direct-connected Have own measuring element in contact with the process; measuring elementscontrollers: available include pressure, differential pressure, temperature, level, pH, thermocou-

ple, radiation pyrometer, and humidity.

Typical Front Panel Size: Miniature: 6 in. × 6 in. (150 mm × 150 mm)

High-density miniature: 3 in. × 6 in. (75 mm × 150 mm)

Large case: 15 in. × 20 in. (375 mm × 500 mm)

Minimum Response Level: Less than 0.01% of full scale

Input Range 3 to 15 PSIG (0.2 to 1.0 bar) or direct-connected(measurement):

Output Range 3 to 15 PSIG (0.2 to 1.0 bar)(signal to valve):

Repeatability: ± 0.2% of span

Inaccuracy: ± 0.25 to 2% of span

Displays: Set point, process variable (measurement), controlled variable (output to controlvalve), deviation, and balance

Maximum Frequency Flat to 30 HzResponse:

Maximum Zero 750Frequency Gain:

Control Modes: Manual, on-off, proportional (P), integral (I-reset), derivative (D-rate), floating, dif-ferential gap, follow-up, cascade

Costs: Direct-connected, indicating, field-mounted controller costs range from $500 to$2000; miniature indicating controller costs $1000 to $2500; miniature high-densityindicating controller indicating $1500 to $2500; miniature recording controller costs

Controlvalve

ToXC

XC

Measurementdirect

connectedcontroller

Set point

Measurementreceiver

controller

Flow sheet symbols


3.1 Controllers—Pneumatic 461

$2000 to $3000; large-case indicating controller costs $1500 to $2500; and large-case recording controller costs $2000 to $3000

Partial List of Suppliers: ABB Group (www.abb.com)Ametek (www.ametek.com)Barton (www.barton-instruments.com/index2.php)Bristol Babcock (www.bristolbabcock.com)Emerson Process Management (www.emersonprocess.com)Foxboro Co. (www.foxboro.com/m&i/specifications/controllers/)Honeywell Automation and Control (hbctradeline.honeywell.com/Catalog/Pages/

default.asp)Leslie Controls, Inc (www.lesliecontrols.com)Powers Process Controls (www.powerscontrols.com)Samson AG (www.samson.de/pdf_en/_ek16_re.htm)Spence Engineering Company, Inc. (www.spenceengineering.com/Handbook/

index.htm)Trautomation (www.trautomation.com/automation/categorie.nsf/)

HISTORY AND DEVELOPMENT

Pneumatic controllers were first introduced at the turn of thetwentieth century. They logically followed the development ofdiaphragm-actuated valves in the 1890s. Early types were alldirect connected, local mounting, indicating, or blind types.Large-case indicating and circular chart recording controllersappeared around 1915. All early models incorporated two-position, on/off action or proportional action. It was not until1929 that reset action was introduced. Rate action followedin 1935.

Until the late 1930s, all controllers were direct connectedand therefore had to be located close to the process. Pneu-matic transmitters were not introduced until the later 1930s.To make them compatible, the large-case pressure recordingand indicating controllers were easily converted into receivercontrollers. This made remote mounting practicable, and cen-tralized control rooms became a reality. Because of the inher-ent advantages, the combination of pneumatic transmittersand receiver controllers quickly became popular. Since therecording and indicating receiver controllers were quite large,control rooms and panel boards were likewise spacious.Additionally, all control boards had a monotonous look andusually came in one color—black.

A revolution in design occurred in 1948 with the introduc-tion of miniature instruments. Here the concept of the control-ler evolved into a combination of a small, approximately 6 in.× 6 in. (150 mm × 150 mm), panel front indicating and record-ing control station and a blind receiver controller. The stationpermitted the operator to monitor the measured variable (pro-cess variable), set point, and valve output; it allowed the oper-ator to switch between, and operate in, either the automatic ormanual control modes.

Miniature controllers ushered in the era of the graphicpanel, in which the instruments are inserted into graphic sym-bols representing the attendant process apparatus. Controlrooms became more compact, control boards more meaningful

and colorful, and because operators quickly developed a “feel”for the process, training time was considerably reduced.

Nevertheless, graphic panels were also wasteful of spaceand presented major modification problems each time theprocess was changed. This led to the evolution of the semi-graphic panel, in which a graphic symbol diagram of theprocess appeared above the miniature instruments mountedin neatly spaced rows and columns.

In 1965, miniature, high-density mounting style stationsappeared. The new lines brought with them the mostadvanced ideas in displays, operating safety and simplicity,packaging, installation simplicity, and servicing facility.Along with some of the standard miniature controllers, theyoffered computer compatibility along with some unique con-trol capabilities that had previously been impractical.

The early 1980s saw some important new entries in thepneumatic controller market. These included pneumatic control-lers using RTD and thermocouple type sensors and pneumaticcontrollers with microprocessor-based serial communicationmodules for tie-in to distributed control systems.

As first analog (Section 3.2), and later digital electronic(Section 4.4) and DCS-based software controllers (Chapter 4)became available, they gradually took over the controllermarketplace, but pneumatic units are still used in manyexisting plants and in locations where intrinsic safety isessential.

OPERATING PRINCIPLES

There are both force and motion balance pneumatic control-lers. A receiver-type pneumatic controller based on the forcebalance principle (moment-balance) is shown schematicallyin Figure 3.1a. A process transmitter (lower left) senses themeasured variable (i.e., process variable, e.g., pressure,temperature, flow) and transmits a proportional air pressureto the measured variable (MV) bellows of the controller.


http://www.abb.com

http://www.ametek.com

http://www.c-a-m.com

http://www.bristolbabcock.com

http://www.emersonprocess.com

http://www.foxboro.com

http://www.lesliecontrols.com

http://www.powerscontrols.com

http://www.samson.de

http://www.spenceengineering.com

http://www.spenceengineering.com

http://www.trautomation.com


The controller compares the measured variable against theset point (SP) and sends a corrective air signal (error ordeviation) to manipulate the control valve, thereby complet-ing the feedback control loop.

Figure 3.1a depicts a so-called direct acting controller;the output of the controller increases when the measuredvariable increases. The controller action is selected as a func-tion of the failure position of the control valve and the rela-tionship between the controlled and manipulated variables.To illustrate this relation with an example, visualize a processcooler. The purpose of the control loop on that cooler is tomaintain the temperature at the outlet of the cooler. If thecooling water valve fails closed, a rise in the detected tem-perature will require further opening of the coolant valve,which means that the air signal to the valve should drop andtherefore the controller has to be reverse acting.

In Figure 3.1a, the controller consists of two sets ofopposed bellows of equal area, acting at opposite ends of aforce beam that rotates about a movable pivot. Extendingfrom the right end of the beam is a flapper that baffles thedetector nozzle of the booster relay.

Booster Circuit

Supply air connects to the pilot valve of the booster andflows through a fixed restriction into the top housing andout the detector nozzle. The flapper is effective in changingthe backpressure on the nozzle as long as the clearance iswithin one-fourth of the nozzle diameter. The restrictionsize is selected on the basis that the continuous air con-

sumption will be reasonable and that it will be large enoughnot to clog with typical instrument air. The nozzle, on theother hand, must be large enough that when the flapper hasa clearance of one-fourth nozzle diameter, nozzle backpres-sure drops practically to atmospheric. It must not be solarge, however, that the seating of the flapper becomes toocritical.

A typical size of restriction is 0.012 in. ID (0.3 mm)while the nozzle would be 0.050 in. ID (1.3 mm). The nozzlebackpressure is a function of flapper position. (For a moredetailed discussion of circuits, refer to Section 1.4, “Elec-tronic versus Pneumatic Instruments.”)

The exhaust diaphragm senses nozzle backpressure andacts on the pilot valve. If the backpressure increases, it pushesdown on the valve, opening the supply port to build up theunderside pressure on the diaphragm until it balances thenozzle backpressure. If the backpressure decreases, the dia-phragm assembly moves upward, allowing the valve to closeoff the supply seat while opening an exhaust seat in the centerof the diaphragm. This allows the underside pressure toexhaust through the center mesh material of the diaphragmassembly until the pressures balance.

Proportional Response

Since a pressure range of 3 to 15 PSIG (0.2 to 1.0 bar) is analmost universal standard for representing 0 to 100% of therange of a measured variable, a set point, and the output ofreceiver controllers, this description will assume the opera-tion to be in this range.

FIG. 3.1aMoment-balance controller.

Air supply

Set pointregulator

SP

A B

PI

PI PI

MV Proportionalband

adjustment

SP - Set pointMV - Measured variableR - Reset FB - FeedbackAS - Air supply

ProcessTransmitter

BoosterRestriction

Derivativeneedlevalve

Controlvalve

FB

R Flapper

AS Exhaust

Detectornozzle

Reset needlevalve

XT



To understand the proportional response, first assume thatthe derivative needle valve is wide open and that the resetneedle valve is closed with 9 PSIG (0.6 bar) mid-scale pres-

point is adjusted to 9 PSIG (0.6 bar), when the measuredvariable equals set point, the flapper will automatically bepositioned so that the booster output, acting on the feedbackbellows (FB), will be equal to the reset pressure, namely 9PSIG (0.6 bar). The reason for this is that the force beamwill only come to equilibrium when all of the moments causea rotation of the beam with attendant repositioning of theflapper and change in feedback pressure until the momentbalance is restored.

If the pivot is positioned centrally, where moment arm Aequals B, then for every 1 PSI (0.067 bar) difference betweenset point and measured variable there will be a 1 PSI (0.067bar) difference between reset and controller output, or feed-back. This represents a 100% proportional band setting, or again of one. Percent proportional band is defined as the inputchange divided by the output change times 100. Proportionalbands are typically adjustable from 5 to 500%. Gain equalsthe ratio of output change to input change. For a descriptionof both the proportional band and of controller action, referto Figure 3.1b.

3.1(1)

= 100/PB 3.1(2)

point where moment arm A is four times greater than momentarm B, then every 1-PSI (0.067-bar) change in the measured(also called controlled) variable results in a 4-PSI (0.27-bar)change in the output (also called manipulated) variable. Thisgives a proportional band of 25%, or a gain of four.

If the pivot is moved to the left, the ratio is reversed andthe proportional band would be 400%, which corresponds toa gain of 0.25. If the pivot could be moved to the right tocoincide with the center of the R and FB bellows, the mostsensitive setting would be achieved, i.e., approaching 0%band or infinite gain. In this state, the slightest differencebetween MV and SP would rotate the flapper to change theoutput to 0 PSI (0 bar) or full supply pressure, dependingupon the action of the error (on/off control).

Reset or Integral Response

If it were practical to use a 1 or 2% proportional band on allprocesses, proportional action alone would be sufficient formost processes. However, most process loops become unsta-ble at such or even wider bands. Flow control loops, forexample, usually require more than a 200% band for stability.Assuming that the process cannot tolerate a band narrowerthan 50%, in that case (Figure 3.1b), the controller can main-tain the measured variable on set point only when valvepressure is 9 PSIG (0.6 bar).

If the valve pressure has to be 5 PSIG (0.3 bar), it canonly occur when the measured variable deviates from setpoint by 2 PSI (0.14 bar) (a 16.7% of full scale error). Inmost cases this amount of error, more correctly termed offset,is intolerable. Nevertheless, in real control systems the valvepressure must change as the load changes.

If the load is such as to require a 5-PSIG (35-kPa) valvepressure, one way of eliminating the offset would be tomanually change the reset pressure R to 5 PSIG. The outputor valve pressure would then be 5 PSIG when the error iszero. Initially, when proportional-only controllers were used,such manual reset was used, but that is totally unacceptabletoday.

It is a simple matter to make this reset action automatic.It only requires that the reset bellows be able to communicatewith the controller output pressure through some adjustablerestriction such as a needle valve. The reset action must betuned to the process in such a way as to allow the processsufficient time to respond. Too fast a reset speed, in effect,makes the controller “impatient” and results in instability. Tooslow a speed results in stable operation, but the offset is per-mitted to persist for a longer period than necessary.

To describe the automatic reset action, assume that setpoint is at 9 PSIG (0.6 bar) and the reset is trapped at 9 PSIG,while valve pressure feedback, because of load change, mustbe at 5 PSIG (0.3 bar). According to Figure 3.1b, with theproportional band at 50%, the measured variable will becontrolled at 7 PSIG (0.46 bar).

FIG. 3.1b Graphic representation of control functions.

PBchange in inputchange in output

% = × 100

gainoutput changeinput change

=

15

14

13

12

11

10

9

8

7

6

5

3

4

3 4 5 6 7 8 9 10 11 12 13 14 15

Reset adjustment

Inversederivative action

Setpointoffset

Directderivative

action

100% PB

50% PB

200%

PB

Output signal to valve (PSIG)*

Mea

sure

d va

riabl

e pre

ssur

e (PS

IG)*

0% PB

*1 PSIG = 0.067 bar


sure, trapped in the reset bellows R (Figure 3.1a). If the set

If the pivot in Figure 3.1a is shifted to the right, to the


If the reset needle valve is then opened slightly, the pres-sure in the reset bellows will gradually decrease. As it dropsfrom 9 to 7 PSIG (0.6 to 0.46 bar), the measured variablepressure will rise from 7 to 8 PSIG (0.46 to 0.5 bar). Thereset pressure will continue to drop until it exactly equalsoutput, i.e., 5 PSIG (0.3 bar). At this point, the measuredvariable will exactly equal set point, 9 PSIG (0.6 bar). Withthis configuration, regardless of where the valve pressuremust be, the controller will ultimately provide the correctvalve pressure with no offset between set point and measuredvariable.

Reset action can also be understood by looking at it fromthe controller design perspective. We can see that the con-troller cannot come to equilibrium as long as there is anydifference in pressure between reset and feedback. This isbecause if there is, the open communication through the resetneedle valve will cause the reset to continue to change, whichin turn directly reinforces the feedback pressure.

Reset and feedback, in turn, cannot be equal unless theset point and the measured variable are equal to each other.This fact alone ensures that the controller will maintain cor-rective action until it makes the measured variable exactlyequal to set point regardless of where feedback must be.Referring to Figure 3.1b, reset in effect shifts the proportionalband lines along a horizontal axis at set-point level. It makesthe middle of the band coincide with the required valvepressure.

Reset time is the time required for the reset action toproduce the same change in output that the proportionalaction would if the error remained constant (the availableintegral time settings range from 0.01 to 60 minutes perrepeat). For example, if the response to an error of a plainproportional controller is a 1-PSI (0.067-bar) change in itsoutput and the error is sustained, the integral mode with a 1minute/repeat reset setting will require 1 minute to eliminatethat offset. The integral setting can have the units of eithertime/repeat or repeats/unit of time.

Derivative Response

If a needle valve is inserted between the booster output and

ancing action of the feedback bellows and causes the con-troller to give an exaggerated response to changes in themeasured variable. The degree of exaggeration is in propor-

changing. (The term derivative action refers to the slope orthe rate of change.)

Derivative or rate action is particularly effective in thecontrol of slow processes, such as in temperature controlloops. It compensates for lag or inertia. For a sudden changeof even small magnitude, it provides an extra “kick” to thecontrol valve. This is because the derivative mode assumesthat upsets will continue at the rate they are occurring andcorrects right away for the error that would evolve one deriv-ative time later. Conversely, when the error is dropping as the

controlled process variable is returning to set point, the rateaction anticipates that the inertia of the process will carry itbelow the set point and begins cutting back the valve responseaccordingly.

action to the has some serious limitations. If the derivativeneedle valve is located as shown, the derivative action willinteract with both the proportional and the integral responsesand, in fact, will follow the proportional response. Thisdesign therefore is useless in preventing overshoot on startupor when large upsets occur, and this derivative also interactswith set point changes. Therefore an independent derivativeunit is needed, as shown in Figure 3.1e and described later.

Derivative action can be viewed as if it temporarilychanged the proportional band and therefore temporarilychanged the slope of the lines in Figure 3.1b. If the error isrising, the change is clockwise; if it is dropping, the changeis counterclockwise; if the error is constant, no changeoccurs. Derivative time is the amount of time (usually inminutes) by which the rate action anticipates into the futureand leads the feedback pressure during a steady ramp inputchange. (Derivative settings can usually be adjusted from0.01 to 60 minutes.)

MINIATURE CONTROLLER DESIGNS

Two designs of force-balance receiver controllers are shownin Figures 3.1c and 3.1d. The controller in Figure 3.1c closelyresembles that of Figure 3.1a, except that the bellows all actfrom one side against a pivoted “wobble plate.” The wobbleplate acts as the nozzle’s baffle. Rotating the pivot axis

FIG. 3.1c Typical moment-balance controller.

Restriction

Nozzle

Air supplyFloating

disc FulcrumsRelay

Feedback

Measuredvariable

Setpoint

ResetTank

Needlevalve

To controlvalve


The method shown in Figure 3.1a for adding derivative

tion to the speed or rate at which the measured variable is

the feedback bellows as in Figure 3.1a, it delays the rebal-


changes the proportional band. When the pivot axis coincideswith the reset and feedback bellows, 0% proportional bandresults; when it coincides with the measured variable and setpoint bellows, infinite band results. When the axis bisects thetwo bellows’ axis, 100% proportional band results. Other-wise, the operation is the same as described for Figure 3.1a.

The controller in Figure 3.1d is constructed of machinedaluminum rings, which are separated by rubber diaphragmswith bolts holding the assembly together. The lower portionof the controller forms the booster section. This is quitesimilar to the booster in Figure 3.1a.

The detector section consists of three diaphragms. Theupper and lower diaphragms have equal areas, while thecenter diaphragm has half the effective area of the other two.Reset pressure acts in the top chamber. Assume this pressureis at mid-scale and that the reset needle valve is closed. Thereset pressure acts on the top diaphragm, which is part of a1:1 repeating relay. Supply air passes through a restrictionand out the exhaust nozzle. The diaphragm baffles the nozzleto make the backpressure equal to the reset pressure.

Assume further that the proportional band needle valveis closed. The pressure then acting on top of the detectorsection will be the reset pressure as reproduced by the 1:1relay since the two chambers are connected via a restriction.If the measured variable signal equals the set point, the detec-tor diaphragm assembly, with its integral nozzle seat, willbaffle off the nozzle so as to make the controller output, orvalve pressure, equal to the reset pressure, thus balancing allof the forces acting on the detector.

If the measured variable is then increased by 1 PSI (0.067bar), the increase in pressure acts downward on the lowerdiaphragm as well as upward on the center diaphragm. Thenet effect is the same as having the pressure act downward

on a diaphragm of half the area of the lower diaphragm. Sincethe output pressure acts upward on the full area of the lowerdiaphragm, it needs to increase only 1/2 PSI (0.03 bar) tobring the forces to balance. Since a 1-PSI (0.067-bar) changein input resulted in a 1/2-PSI change in output, the propor-tional band is said to be at 200% or the gain is said to be 0.5.

If the proportional band needle valve were wide open, sothat it provided negligible resistance, then if the measured vari-able pressure increased ever so slightly above set point, the fulleffect of the resultant change in output would be felt on top ofthe detector stack. This would cause the output to increasefurther, which in turn would feed upon itself, and the actionwould continue to regenerate until the output reached its max-imum limit. Therefore, with the proportional band needle valvewide open, the narrowest proportional band is obtained.

If the proportional band needle valve is set to where itsresistance equals that of the restriction separating the repro-ducing relay from the top of the detector section, then thefollowing action results. If the measured variable deviatesfrom set point by 1 PSI (0.067 bar) in an increasing direction,instantly the output will rise 1/2 PSI (0.03 bar) because ofthe construction of the detector section. The difference inpressure between the controller output and the reproducingrelay will cause a flow through the reset needle valve and theintermediate restriction.

Since the resistance of the two is equal, the pressuredrop will divide equally, causing a 1/4-PSI (0.017-bar) increaseon the top of the detector section. This 1/4-PSI increasedirectly causes the output to increase 1/4 PSI, which furthercauses the pressure on top of the detector to increase by1/8 PSI (0.008 bar). The action continues until equilibriumis obtained, with the output having changed a total of 1 PSI(0.067 bar) and with the pressure on top of the detectorsection having increased 1/2 PSI (0.03 bar). Since a 1-PSIchange in variable resulted in a 1-PSI change in output, thisneedle valve opening provides a 100% proportional band.

The reset action in this controller is similar to that inFigure 3.1a in that any change in reset pressure propagatesdown through the unit to directly affect the output. Thecontroller will not come to equilibrium until all forces arebalanced, i.e., the measured variable will have to equal setpoint and the reset pressure will have to equal controlleroutput.

Derivative Relay

It was noted earlier that the type of derivative circuit used inFigure 3.1a interacted with the effects of the other controlmodes. Derivative action is more effective if it is noninter-acting and if it can be applied ahead of the contributions ofthe proportional and reset actions of the controller. Such aderivative unit can be built into the controller, or it can be aseparate relay as shown in Figure 3.1e. This relay employsdiaphragms, but the design can also use bellows.

In this design the signal from some process transmitter isconnected to the input port at the top. Because of the difference

FIG. 3.1d Force-balance controller.

To valve

Exhaust

Detectorsection

Proportionalband valve

Resetfeedback

switch

Mainvalveload

Reset valve

Automatic

Manual

4681020

12

.5

.2

.1

Air supply

Measuredvariable

Set point

Restriction



between input and output diaphragm areas, the result of theforce balance design is that a step change in input pressure willproduce an amplified step change in output pressure.

In a steady-state condition (with no change in input), theoutput pressure acts on both sides of the output diaphragm —and output pressure rebalances the input pressure directly.The output pressure is connected to the intermediate chamberthrough the needle valve. There is therefore a lag between achange in output and a change in intermediate pressure.

If the input changes with a continuous ramp, the inter-mediate pressure will lag the output by a constant amount,proportional to the rate of change in output. Thus, the inter-mediate pressure will partially rebalance the input, reducingthe effective gain. The result is that the output will continu-ously lead the input by a definite amount, which is propor-tional to the rate of change in input. The time by which theoutput leads the input is the “derivative time.” The graduatedneedle valve is used to set the derivative time.

An inverse derivative relay is also shown in Figure 3.1e.It works in the opposite manner and thereby attenuates high-frequency signals. It can therefore serve as a noise filter andstabilizing relay on “noisy” processes.

Miniature Control Stations

Miniature control stations with a panel face of nominally 6 in.× 6 in. (150 mm × 150 mm) and inserted into individual cutoutshaving approximately 10 in. (250 mm) center-to-center dis-tances are one of the common types found on central controlroom panels in the various process industries. Figure 3.1f showsa typical cross section of some of the types of units available.

FIG. 3.1e Direct and inverse derivative relays.

.2

.5123

Needle vlave

Input

Intermediatechamber

Detectingnozzle

Restriction

Exhaust

Supply Output

Direct- derivative

Derivative- timeadjustment

.2

.5125

Derivativetime

Input

Output

Ideal

TimeDirect - derivative response

Pres

sure

Inverse - derivative

FIG. 3.1fTypes of miniature pneumatic controllers. (a) Typical indicating control station. (b) Indicating control station with 12 o’clock scanningfeatures. (c) Recording control station with 30-day strip-chart and vertical moving pen. (d) Recording control station with horizontal movingpen and daily chart tear-off feature. (e) Indicating control station with two duplex vertical scale indicators (f) Recording control stationwith no “seal” position. (g) Recording control station with servo-operated pen. (h) Recording control station with procedureless switching.(i) Indication control station with instant procedureless switching.

0

20

40 60

80100 40

60

1000

(a) (c) (d)(b) (e)

(f ) (g) (h) (i)

0

20

40 60

80

100



The interconnections of an indicating miniature controlstation are shown in Figure 3.1g. The measured variable isindicated on the center pointer, and the set point is indicatedon the peripheral pointer of a duplex gauge. On automatic,the operator changes the set point by adjusting the set pointregulator and noting the set point on the gauge.

The controller is connected to the control valve via themanual–automatic switch. The controller compares the mea-sured variable with the set point and manipulates the controlvalve to bring the variable on set point. If the operator wishesto switch to manual, he or she notes the valve pressure byoperating the upper left-hand switch on the station. Next, theoperator turns the right-hand switch to “seal,” which isolatesthe controller from the control valve. After that, the operatoradjusts the regulator to match the noted valve pressure andthen turns the right-hand switch to the manual position, whichconnects the regulator directly to the valve. In manual, theoperator directly adjusts the valve, while the controller resetfollows the changes that are made to the valve.

In switching back to automatic, the operator goes to“seal” position and adjusts the set point regulator to matchthe measured variable. If the set point and measured variable

are equal at switchover, and if the reset is equal to the valvepressure, the controller output should then equal the valvepressure, and the switchover is effected without a “bump.”

Four-Pipe System Since a lag exists in the transmission ofpneumatic signals, the dynamic capability of a control loopcan be affected by increasing the distance between the con-troller and the process. Transmission lag will interfere withthe performance of fast control loops such as liquid flow con-trol but will not be significant if the distance is up to 100 ft(30 m).

For control loops where the transmission lag cannot betolerated, the controller can be mounted locally, near the mea-suring transmitter and control valve, as shown in Figure 3.1h.

In this design, four air signal tubes are run between thecontrol station and the field-mounted equipment. These carrythe measured variable, set point, valve pressure, and relayoperating pressure lines. Hence, the name “four-pipe system,”whereas the configuration in Figure 3.1g is referred to as a“two-pipe system.” Since the lines going back to the stationamount to dead-ended parallel connections, the dynamics of

FIG. 3.1gMiniature control station with integral-mounted controller.

CRegulator

Control stationfield

Processtransmitter Control

valve

M/Aswitch

COMV Auto

RSP

Reg.

Valve

S

S

Auto Seal Manual

M/A switch positions

S - Air supplySP - Set pointMV - Measured variableCO - Controller outputR - Reset feedbackC - Controller Mechanical connection

Pneumatic connectionM/A - Manual-automatic

XT

FIG. 3.1hMiniature control station with field-mounted controller (four-pipesystem).

XT

Auto Seal ManualSwitch No. 1

Switch No. 2

SwitchNo.2

SwitchNo.1

Vent

Switch positions

RegulatorS Plug

Control stationfield

SP R

COMV C

Air to closedrelay

SProcess

transmitter Controlvalve

S



the control loop are the same as they would be with anyclosed-loop system.

In switching from automatic to manual, the operatorswitches the loop to the “seal” position, which actuates thecutoff relay, to isolate the controller from the valve and per-mits the operator to change the set point regulator to matchthe noted valve pressure. This pressure is then connected tothe valve when the operator turns the switch to manual.Returning to automatic involves the same procedure as wasdescribed for Figure 3.1g.

The disadvantages of this circuit are that:

1. It is more costly to run four transmission tubes betweenthe control station and the field-mounted equipment.

2. The controller settings cannot be adjusted from thecontrol panel.

Effect of Transmission Distance Since there is a transpor-tation lag caused by pneumatic transmission, control isaffected as the distance between the process and controllerincreases. Figure 3.1i shows the effect of increasing trans-mission distance between the process and the controller. As

can be seen, a 10% upset in liquid flow will result in longerand longer recovery times as the transmission distanceincreases. Since liquid flow control is one of the fastest pro-cesses, this amounts to a worst-case example; the effect onslower processes will be proportionately less.

From the charts it can be seen that with all instrumentsclose-coupled it took 8 seconds for the system to recover. Ata transmission distance of 250 ft (75 m), the recovery timewas still approximately 8 seconds. At 500 ft (150 m), it was16 seconds, and at 1000 ft (300 m), 37 seconds. These resultswere obtained using 1/4 in. (6 mm)-OD tubing, which isconventional.

When 3/8 in. (9 mm) tubing was used, there was a sig-nificant improvement. At 1000 ft (300 m), the recovery timewas reduced from 37 to 27 seconds. An equivalent electroniccontrol loop had an 8-second recovery time. The upper, noisyrecord on the charts (b), (c), (d), and (e) is a plot of the flowas it was recorded at the transmitter. The lower, smootherrecords are the flows as they appeared on the remotely locatedrecorder-controller.

If this lag is objectionable, the four-pipe system shownin Figure 3.1h can be used, and if it was, the dynamic per-formance would be equivalent to that of the closed-loopsystem. Other options include the installation of boosterrelays in the transmission lines or the use of larger-diametertransmission tubing.

Remote-Set Stations If a modification as shown in Figure 3.1jis added to the basic station in Figure 3.1g, the control stationcan accommodate a remote set point signal from sources suchas remote ratio or proportioning relay, primary cascade con-troller, or computer.

Computer-Set Station The addition of a stepping motor tothe set point regulator or to the set point motion transmitteras in Figure 3.1k allows the control station to be set from adigital computer. The station provides the option of switchingthe loop from computer control of the set point to manual,as shown in the Figure. The stepper motor, which operatesthe regulator, can be driven by time-duration signals or byindividual up-down pulses. A resolution of at least 1000pulses for full scale is provided.

FIG. 3.1i Effect of transmission distance on control of a liquid flow controlprocess (worst-case example) with 10% step upset. Upper noisycurves show flows recorded locally; lower smooth curves show flowsrecorded remotely at controller. (J.D. Warnock, “How PneumaticTubing Size Influences Controllability,” Instrumentation Technol-ogy, February 1967.)

8 sec− close coupled

16 sec

500 feet(150 m)

1,000 feet(300 m)

37 sec

27 sec

1,000 feet(300 m)

3/8−inch(9 mm)Tubing

8 sec

250 feet(75 m)

Flow Fl

ow

Flow

Flow

10 20 30 40 50 600

10 20 30 40 50 600 10 20 30 40 50 600

10 20 30 40 50 600

10 20 30 40 50 600

Time, sec

Time, sec

Time, sec

Time, sec

Time, sec

(a) (b)

(c) (d)

(e)

Flow

FIG. 3.1j Control station circuit for remote set point adjustment.

C

S

RegulatorSwitch

Remoteset pointsource

Remoteset

Localset

SP

MV

S

R

CO



Single-Station Cascade Cascade control can be imple-mented either with two controller stations, with the mastercontroller (such as Figure 3.1g) generating the set point forthe slave (or secondary), which is connected as in Figure 3.1j.The other option is to use a single station, such as shown inFigure 3.1l. The latter scheme not only eliminates one of thetwo stations but also offers operating safety and convenience.

A common problem with having separate master andslave stations is that when the operator switches the slave tomanual, he or she often forgets to also switch the masterstation to manual as well. In such situation, the primarycontroller has no influence on the manipulated variable andwill not be balanced when the loop is returned to cascade.This is particularly undesirable because cascade circuits areusually used on the most critical loops.

In Figure 3.1l, the regulator has three functions:

1. Set point to primary controller in cascade control2. Set point to secondary controller for independent sec-

ondary control 3. Manual valve setting

There is a seal position between each step while the reg-ulator is set for its upcoming function. The key to this stationis the concept used in making the secondary measured variable(MV2) the reset feedback of the primary controller while onmanual or secondary control. Versions are also available thatallow cascade, independent automatic control on the primaryand manual modes of operation.

“No Seal” Station By using two regulators or two motiontransmitters in a station (Figure 3.1m), the need for a “seal”position can be eliminated. In this configuration, when theoperator wishes to switch to manual, he or she adjusts themanual regulator to match the controller output while viewinga deviation indicator. When the deviation is zero, the operatortransfers control.

Procedureless Switching Stations With procedureless switch-ing, the operator simply turns a switch, and the station automat-ically takes care of the pressure-balancing requirements. Twodesigns are shown in Figure 3.1n. The mechanism on the top(a) is a motion transmitter/receiver combination. This motiontransmitter provides the set point pressure when the controlleris in automatic and the valve pressure when in manual—similarly to the regulator in Figure 3.1g.

As a motion transmitter, a friction clutch holds the indexlever at whatever position the operator has set it. A restriction-nozzle circuit senses the position and converts it to a propor-tional pneumatic output that is fed back to the rebalancingbellows. A 0 to 100% movement of the index gives a 3- to15-PSIG (0.2- to 1.0-bar) output pressure. When acting as areceiver, supply pressure is cut off from the restriction nozzlecircuit and the pressure to be sensed is admitted to the rebal-ancing bellows.

The friction clutch is disconnected so that the indexlever can be moved by the rebalancing bellows. The unit isso designed and calibrated that a 3- to 15-PSIG (0.2- to 1.0-bar) sensed pressure produces a 0 to 100% index movement.Therefore, as the operator moves the switch from automatic

FIG. 3.1kControl station circuit for computer-adjusted set point control.

FIG. 3.1l Single-station cascade controller.

CCO

R

M

Computer

SP

MV

Controlstationmanual

set pointadjustment

Regulator or motion

transmitter

S

C1

C2

Regulator

SP1 R1

CO1MV1

Cascade Manual

R2SP 2Secondary

CascadeMV 2 CO2 Auto

To valve

Manual

FIG. 3.1mThe use of two-regulator stations eliminates the need for a ‘‘seal’’position.

Regulator

CDeviationindicator

ManualAuto

To valve

S S

RSP

MP

Regulatoror motion

transmitter



to manual, the following actions take place in sequence andautomatically:

The index is declutched; supply is cut off from the restric-tion nozzle circuit; controller output (valve) pressure is admittedto the bellows causing the index level to take a position pro-portional to the pressure; controller output is disconnected fromthe valve line; the clutch is engaged; supply pressure is read-mitted to the restriction nozzle circuit; and the unit again actsas a motion transmitter, now providing valve pressure.

Switching back to automatic involves the same sequence,in reverse order. This is the same sequence as carried out inFigure 3.1g, except that here it is automatic.

A second system for procedureless switching involvesthe use of self-synchronizing regulators or synchros (seedesign b in Figure 3.1n). One regulator provides set point,while the other is used for manual valve loading. As thename implies, this regulator can synchronize itself to some

varying pneumatic pressure and thereby provide automaticbalancing.

The regulator employs a reaction nozzle circuit that resultsin very low spring force (approximately 1 oz or 0.28 N) todevelop a 3- to 15-PSIG (0.2- to 1-bar) output. The settingspring is adjusted by rotation of a turbine wheel with anintegral lead screw (part b in Figure 3.1n). If supply is con-nected to the comparator controller section, air is transmittedto the increase–decrease nozzles to make the regulator sectionoutput match the variable input pressure. If supply is cut offfrom the comparator controller, the regulator section outputremains locked in, with the memory being a function of leadscrew position. The unit can then be driven manually by theoperator.

When this control station is in automatic, the set pointsynchro is manually adjusted by the operator, while the valve-operating synchro keeps itself matched to the controller outputto allow instant transfer to manual. In manual, the operatoradjusts the valve-loading synchro while the set point synchrotracks the measured variable.

In addition to procedureless switching, these stations canalso be switched from a remote source, manually or auto-matically. They can also be gang-switched, and they can beoperated in parallel. For example, it is possible to have onestation in the central control room and the other out in thefield. The operator can use either station and whichever isnot in active service keeps itself fully synchronized and readyto take active duty at any instant.

High-Density Stations

Miniature high-density stations have a typical panel size of3 in. × 6 in. (7.5 mm × 150 mm), mount adjacent to eachother, and allow very compact and efficient panel arrange-ments (Figure 3.1o). The units incorporate packaging featuresthat simplify panel construction and design and facilitateservicing. Much of the design is aimed at making the job ofthe operator simpler, faster, and safer, in line with the presenttrend to consolidate control rooms, minimize the number ofoperators, and handle increasingly fast, complex, and criticalprocesses.

Mid-Scale Scanning Station Figures 3.1p, 3.1q, and 3.1rshow three types of high-density control stations featuring amid-scale deviation scanning pointer. The pointer is driveneither by a differential detector or differential servo that com-pares the measured variable against set point. If the two areequal, the red deviation pointer is positioned at mid-scale,where it is screened off by a green scan band. If there is adeviation, the red pointer stands out prominently.

In Figure 3.1p, a fixed, nominal 4 in. (100 mm) verticalscale is employed, and there are separate pointers to indicateset point and measured variable. The station uses the two-regulator approach to achieve “no-seal” switching as inFigure 3.1m. The operator does have to balance pressuresbefore switching, however.

FIG. 3.1nTwo approaches to self-balancing and to procedureless switchingin control stations.

15

9

3

Boosterrelay Set point or

valve loadingAlternating motion transmitter/receiver

Motiontransmitter Receiver

Knifeedge

Rebalancingfeedbackcapsule

Indexlever

Nozzle

Flapper

Cam

Cam

Clutch

Measuredvariable

Valvepressure

(a)

S

Variable

Increase outputDecrease output

OutputSpring

Regulatorsection

Comparatorcontroller

Turbinewheel

Manual pushbuttonsReactionnozzles

S

S

Self-synchronizing regulator(b)



In Figure 3.1q, the station employs an expanded scale,which provides greater readability. The only indication onthe scale is deviation, however, and this requires that the setpoint transmitter scale and deviation servo stay in calibrationrelative to each other in order to provide an accurate readingof the variable. While the expanded scale gives greater read-ability, it does have to be moved to bring the reading on scalewhen the variable makes any excursions.

This station and the one in Figure 3.1r use the two-regulator approach to eliminate the need for a seal position.In both cases the operator balances pressures prior to switch-over. In Figure 3.1q, the operator notes deviation on a ball-in-tube indicator. In Figure 3.1r, the valve switch has a detentaction while the indicator switch operates at the mid-throwposition, so that the operator moves the integral switch leverback and forth across center while manually matching pres-sures before switching. The controller in Figure 3.1r is adeviation type actuated by displacement of a deviation linkin the indicator circuit. The controller acts to hold the linkat its “zero” position.

While thousands of these control stations have been usedin control rooms, mid-scale scanning stations have evolved

FIG. 3.1oTypical mounting arrangements of high-density control stations.

Relay rack

Console

Semi-graphic

FIG. 3.1pFunctional diagram of high-density station with mid-scale scanningand individual indication of set point and measured variable.

FIG. 3.1qMid-scale scanning station with expanded scale.

Green

RedS

S

Set pointtransmitter

MV SPDeviationdetector

Controller

Manualtransmitter

Measurement

Manual Auto

To valve

Set pointadjustment

C

MV

Deviationservo

SP

Remoteset point

Tapedriveservo

On cascade stations only

Ball gauge

Manual Auto

Relay

Manualregulator

S

S



into a type combining mid-scale scanning with procedurelessswitching, using principles described in the next paragraphin connection with Figure 3.1t

Procedureless Switching Station Stat ions shown inFigures 3.1s and 3.1t offer procedureless switching for theoperator. Both have a fixed 4 in. (100 mm) scale, separateindication of set point and measured variable, and a scanningtechnique that allows the set point indicator to overlap themeasured variable indicator when the controlled variable ison set point.

The station in Figure 3.1s employs two self-synchroniz-ing regulators or synchros (see Figure 3.1n), one for set pointand the other for manual valve loading. When the controlleris in automatic, the operator manually adjusts the set pointsynchro, while the valve-loading synchro automaticallytracks controller output. In manual, the operator adjusts thevalve-loading synchro while the set point synchro tracks themeasured variable. Thus, the station is always balancedallowing for instant transfer of the control mode.

Like its 6 in. × 6 in. (150 mm × 150 mm) counterpart,these stations can also be gang-switched, switched remotelyor automatically, and operated in parallel from different loca-tions while maintaining themselves in synchronism, and theyare also available in single-station cascade arrangements.

Motion Transmitter/Receiver The station in Figure 3.1temploys a dual function motion transmitter/receiver for man-ual valve loading and valve pressure indication. This unit issimilar to that described in Figure 3.1n. In automatic, theindex lever is declutched and the feedback capsule, connectedto the valve pressure line, moves the index accordingly. Inmanual, the clutch engages, and the mechanism reverts to amotion transmitter that provides manual valve loading. Thisallows procedureless switching to manual.

Switching to automatic is also procedureless, assumingthat the set point of the process does not change. If theoperator wishes the controller to operate at some new value,compared to where the process had been set on manual, theoperator must change the set point to that value prior toswitching to automatic. However, to facilitate switching toautomatic on loops where the set point remains fixed, thestation incorporates a separate balancing controller that oper-ates while the station is in manual.

The balancing controller manipulates controller resetpressure to keep the controller output equal to the manual

FIG. 3.1rMid-scale scanning station, expanded scale, and deviation controller.

FIG. 3.1sSelf-synchronizing control station with procedureless switching.

Manual Auto

C

SManual

regulatorTo valve

ValvegaugeSet point

adjustment

MV

SR1 SR2

SR - Self - synchronizing regulatorS/S - Self - synchronizing activation signal

A M

Fromtransmitter

SP

MV C

AutoS/S

CO

Auto Manual

Tovalve

Manual

FIG. 3.1tSelf-balancing control station.

MT - Calibration matched alternating receiver-motion transmitterBT - Balancing controllerST - Set point transmitter

ST SP

MV

S

CR

COS

SMT

Manual

Manual

Manual

Auto

Auto

Auto

Measurement To valvePlug

BC



valve loading even though the measured variable may be offset point. This allows the operator to switch to automaticwhile off the intended set point and yet have the pressuresbalanced at switchover and therefore the system to return toset point without overshoot.

This feature of the circuit is somewhat limited on narrowproportional band applications, which is usually the case withslow processes. This is because it takes little deviation fromset point before the reset would have to be at either a vacuumor considerably above the air supply pressure to obtain thebalance between valve loading and controller output.

LARGE-CASE DESIGNS

Receiver Controllers

Because of their size, large-case receiver controllers are evenless frequently used as miniature designs. They neverthelessstill find use in some plants and industries where the 24-hourcircular chart is traditional and preferred.

Most large-case controllers operate on a displacement bal-ance principle. The set point is a mechanical index setting. Themeasured variable acts on a pressure spring such as a bellows,spiral, or helix that moves the recorder pen or indicator pointer.A differential linkage detects any deviation between the indexand pen position and actuates the flapper-nozzle system in aneffort to bring the deviation down to zero.

One example of a large-case recording controller is shownin Figure 3.1u. In this design, if the pen moves clockwise, thedifferential link moves upward, and the bell-crank moves theflapper toward the nozzle. The resultant increase in nozzle back-pressure is reproduced by the relay, whose output is connectedto the control valve and the housing of the proportioning bel-lows. The pressure increase is transmitted to the small innerbellows, causing the two connected inner bellows to move tothe right. The spring in the left inner bellows compresses whilethe spring in the right distends.

The motion also causes the large right-hand bellows tomove to the right against the housed spring. As the center rodjoining the inner bellows moves to the right, the flapper ismoved back away from the nozzle. This negative feedbackresults in proportioning action. The greater this negative feed-back, the greater the change that will be required in the mea-sured variable to obtain a given change in the valve. Adjustingthe linkage to change the amount of this negative feedbackchanges the proportional band.

Opening the adjustable restriction between the two largebellows allows flow from the bellows at higher pressure tothe one at lower pressure. In this example, it would flowfrom the left to the right bellows, causing the inner bellowsto move left, moving the flapper toward the nozzle andincreasing output further. This action would continue toregenerate until the pen finally returned to the index, wherefull balance would be achieved with the pressure equal inthe two large bellows and with the two inner bellowscentered.

Large-case controllers are also available with remote air-operated set point adjustment as required in cascade and ratiocontrol.

Direct-Connected Controllers

Direct-connected controllers have their own measuring ele-ments, which, as the term implies, are directly connected tothe process. They therefore eliminate the need for a transmit-ter. However, the fact that direct-connected controllers mustbe located in the vicinity of the process limits their use tomounting on local control panels. With this design, the pro-cess connections must be run to the local control panel, whichis costly, troublesome, and hazardous. For these reasons,these units are seldom used, even on smaller installations andon local panels.

Direct-connected large-case controllers of the indicating andrecording type predated the receiver type units by some 20 years.In the first designs, the receiver controllers were direct-con-nected pressure controllers with a 3- to 15-PSIG range.

The operation of direct-connected large-case controllersis the same as that of large-case receiver controllers. The penor pointer arm, instead of being actuated by pressure from aprocess transmitter, is actuated by its own built-in processpressure detector. The cross sections of some of the measur-ing elements that are available with large-case controllers areshown in Figure 3.1v.

These include sensors for the detection of pressure, absolutepressure, draft, vacuum, differential pressure, liquid level, andfilled systems for temperature measurement. Basic electricalmeasurements involved in thermocouples, resistance bulbs,radiation pyrometers, and pH probes are accommodated in thepotentiometer versions of large-case pneumatic controllers.

As with the large-case receiver controller, the direct-connected ones are also available with remote set pointadjustment for cascade and ratio control applications.

FIG. 3.1uLarge-case recording controller.

Set point (index)adjustment

Adjustablerestriction Differential

linkPen

Nozzle

Relay

Flapper

Output to valve

S



FIELD-MOUNTED CONTROLLERS

Direct-Connected

Direct-connected controllers mounted in the field are smallerthan the large-case instruments. They usually have weather-

proof cases and are available in both indicating and blinddesigns. They can be pipe-mounted, mounted directly on avalve or surface, or flush-mounted on a local panel. Theyinclude their own measuring elements. Figure 3.1w showssome typical types and mounting arrangements.

These units are the least expensive pneumatic controllers,and they are expected to be less accurate than the previouslydiscussed designs are. They find use in small local installa-tions and as local field loops in larger plants. Every plant hassome noncritical loops that do not require auto–manualswitching or the displaying of their controlled variable oraccess to their set point from the control room. These unitsare used as local pressure, temperature, and level regulators.

These field-mounted controllers can be had with on/off,differential gap, proportional, reset, and derivative modes ofcontrol. Their principles of operation are similar to that dis-cussed in connection with Figures 3.1a and 3.1w.

Receiver-Type

Some field-mounted local controllers receive a signal from atransmitter rather than having their own measuring element.Such controllers are used if the measurement must be trans-mitted to the control board for recording, alarm, or indication,but the controller is local. Field-mounted receiver controllersare shown in Figures 3.1c and 3.1d. Remote adjustment ofset point is also an option with these controllers.

Pneumatic with Electronic Detectors

The temperature controller illustrated in Figure 3.1x, receivesa temperature measurement signal from either a thermocoupleor a resistance temperature detector (RTD). These indicatingpneumatic controllers provide the accuracy, convenience, andrange of electrical temperature sensing but without the needfor external electrical power. A built-in generator accepts aconventional pneumatic pressure source and produces electri-cal power for the controller.

The controller compares the process temperature signalwith an operator-adjustable set point and delivers a pneumaticcontrol signal to a final control element, which then moves theprocess temperature towards the set point. Process temperature

FIG. 3.1vDirect-connected large-case recording and indicating controllerscan be provided with a variety of direct sensors, including the shownmeasuring elements.

Recording controller

Indicatingcontroller

Measuring elements

Pressure, absolute pressuredraft, vacuum

Differentialpressure

Temperature(filled systems)

�ermocouple

Liquid levelHumidity

pH

FIG. 3.1wTypical installations of direct-connected, locally mounted controllers.

S

S

S

S

Temperature

Tovalve

Pressure

Flow

To valve

Level



and set point are both indicated on this controller. Thesecontrollers are available with PI and PID control modes andwith or without bumpless and balanceless auto/manualswitching.

In some designs, the air supply is also used to generate therequired electric power for the unit and therefore, the maxi-mum “instantaneous” air supply requirement could reach 80SCFH (2.3 normal m3/hr), while the maximum “steady state”air consumption is only 35 SCFH (1.0 normal m3/hr).

In this controller, a thermocouple or RTD temperaturesensor signal is applied to the circuit board, which electricallycompares the signal with the set point value and acts on theerror with its proportional, reset, and rate control modes torestore the process temperature to the set point value.

SPECIAL PNEUMATIC CONTROLLERS

Connections for Digital Highways

Figure 3.1y illustrates a pneumatic controller, which is pro-vided with microprocessor-based digital highway communica-tion modules for integration into distributed control systems(Figure 3.1y). By thus making it possible to communicateover a data highway, pneumatic controls were made compat-ible with distributed systems.

The serial communications module reports, upon com-mand, the current value of the set point, process variable, andvalve output and the operating mode that the station is in.The communications module can also receive and executecommands, which can change set points or outputs or oper-ating modes. Miniature transducers convert the pneumaticsignals to electric, from which the signals are then convertedto digital. The serial link operates at 19.2 kilobaud.

The first data highways were introduced in the late 1970s.Fieldbus is an architecture that provides a communication linkbetween all instruments and all computers using a standardinterface. Several manufacturers have entered the fieldbus tech-nology market. However, the Internet seriously threatens thefieldbus market, because Web-based communication betweeninstruments and computers may eventually lead to the replace-ment of some fieldbus components.

Special Controls

Feedforward, ratio, cascade, and other loop configurationscan be easily implemented with pneumatic hardware. Two ofthe popular control configurations involve selective and batch

FIG. 3.1xPneumatic controller with electronic temperature sensor.

Operator control panel withcircuit board located behind

Electricalsignal to

circuit board

Temperaturesensor

Electrical powerfor circuit board

Supplypressure

Supplypressure

gauge

Outputpressure

gauge

Pneumaticoutput to

final controlelements

Current outputfrom circuit board

Generator Current-to-pneumatictransducer

FIG. 3.1yPneumatic controller with digital highway communication modulefor use in microprocessor-based distributed control systems.

50

Controller Digital data highwaycommunications

module

Motorizedset point/output

regulator

P/Etransducers

100

0



control. Controllers are sometimes packaged specially forthese applications.

Selective Control With automatic selector control, alsocalled override or limit control, two or more control loops areconnected to a common valve. In this configuration, when theconditions are normal, the normal controller has command ofthe valve. However, if some abnormal condition arises, one ofthe other loops can automatically take over control to keep theplant operation within safe limits.

Unlike safety shutdown systems, here the plant is kept inoperation, although its production rate might be cut back asmuch as necessary to stay within safe limits. When the abnormalcondition abates, the normal loop resumes control. Figure 3.1zshows a control system that can be used on a booster pumpstation that is serving a transcontinental pipeline.

Under normal conditions the discharge pressure of thepump is being controlled. If the suction pressure gets too low,however, as would be the case if the booster pump upstreamfailed or if a line rupture occurred, the discharge controllerwould open the valve wide, which would lower the suctionpressure beyond safe limits, causing cavitation, which couldseriously damage the pump.

To protect against this, a suction pressure controller isadded, which is set to the low safe limit, and a low-pressureselector is installed on the two controller outlets. Since thecontrol valve is air-to-open, the low selector chooses the outputof that controller, which is asking for the valve to be less open.Under normal conditions, when the suction pressure is ade-quate, the output of the discharge pressure controller will bethe lower and hence, it will throttle the valve. If suction pres-sure drops to the set point of the suction pressure controller,it automatically takes over control.

A key requirement for correct implementation of thissystem is to provide both controllers with a live reset feed-back from the valve pressure. In this way the controlling unit

has its reset acting normally while the standby controller isprevented from saturating or winding up. This way, wheneither controller takes over, its reset will exactly match thepressure of the signal reaching the valve at the instant ofswitchover.

Batch Control Figure 3.1aa shows a control system forbatch pressure control. Batch control is special because whena batch is completed and the manual valve is closed whilethe controller is in automatic, the reset mode of the controllerwill keep integrating the error until the controller saturates(winds up).

At the next startup, if the controller is PI only, or if it isa PID controller but its derivative mode is interacting withthe proportional and reset actions (Figures 3.1a and 3.1c),the controller output will stay saturated until the errorchanges sign. Therefore, the controller will stay inactive untilthe process measurement crosses over set point. This willresult in a considerable overshoot.

One solution is to add the antireset windup relay shown inFigure 3.1aa. This is simply a throttling relay set to operate at15 PSIG (1 bar), which corresponds to the wide-open positionof the control valve. As long as the controller output is below

FIG. 3.1zAutomatic override or selector control circuit on pipeline boosterpump.

MV MV

SP

CO CO

R R SP

LPS

PT PT

CS CD

LPS − Low pressure selector relay CS − Suction pressure controller CD − Discharge pressure controller

Booster pump Air-to-open

FIG. 3.1aaAlternative circuits for eliminating over-peaking during startup ofbatch processes.

MV MVSP SP

MV MVSP SP

Overshoot

Time TimeStart-up without

derivativeOvershoot withoutanti wind-up relay

Time TimeStart-up with

derivativeStart-up with anti

wind-up relay

Proportionalband

Proportionalband

Derivativeresponse

Anti-resetwind-up

relay

D. R.

SP

MV

R

CO

Alternative solution:non-interactingderivative aheadof proportionalplus reset action

Air-to-open

Controlvalve

Manualvalve

C

PT



15 PSIG, the relay transmits the output to the reset feedbackconnection of the controller and the reset acts normally.

If the output goes above 15 PSIG, the relay beginsexhausting the reset feedback line until the output pressuredrops to 15 PSIG. Thus it does not affect control except whenthe system is shut down. At that time it lowers the resetpressure to whatever value it has to be in order to limit itsoutput to 15 PSIG. This allows the proportional action to beactive during startup so that it can prevent overshoot. Howeffective the antireset windup protection is depends upon thequality of the tuning of the controller.

An alternative solution to reset windup on batch appli-cations is to use either a separate derivative unit or a controllerwith a built-in derivative unit ahead of the proportional-plus-integral sections. This way the derivative unit’s outputcrosses over the set point sooner than the variable itself does,and this starts the reset to unwind, which is in time to preventovershoot.

The effectiveness of this circuit also depends upon propertuning. Too little derivative allows some overshoot; too muchcauses initial undershoot.

Bibliography

“Analog Controllers,” Measurements and Control, February 1992.“Analogue Pneumatic Signal for Process Control Systems,” IEC 60382,

1991.

Buckley, P. S., “Dynamic Design of Pneumatic Control Loops: Parts I andII,” InTech, April and June 1975.

Buckley, P. S., and Luyben, W. L., “Designing Longline Pneumatic ControlSystems,” Instrumentation Technology, April 1969.

Connel, B., Process Instrumentation Applications Manual, New York,McGraw-Hill, 1996.

“Controllers with Analogue Signals for Use in Industrial-Process Control Sys-tems. Part 1: Methods of Evaluating the Performance,” IEC 60546-1,1987.

Goldfeder, L. B., “Analog Control Architecture: Integral vs. Split,” InTech,April 1977.

Jones, B.E., Instrumentation, Measurement and Feedback, New York:McGraw-Hill Book Co., 1977.

Nachtigal, C. N., Instrumentation and Control, New York: Wiley, 1990.Parr, E. A., Hydraulics and Pneumatics: A Technicians and Engineers Guide,

Oxford: Butterworth-Heinemann, 1999.Patrick, D. R., and Fardo, S. W., Industrial Process Control Systems, Albany,

NY: Delmar Learning, 1997.Pessen, D. W., Industrial Automation: Circuit Design and Components, New

York: John Wiley & Sons, 1989.“Quality Standard for Instrument Air,” ANSI/ISA-7.0.01-1996.“Reset Feedback in Pneumatic Controllers,” Moore Products Co., Spring

House, PA.Seborg, D., et al., Process Dynamics and Control, New York: Wiley, 1989.Senbon, T., Hanabuchi, F. (Eds.), Instrumentation Systems — Fundamentals

and Applications, Berlin: Springer-Verlag, 1987.Shinskey, F. G., “Control Topics,” Publ. No. 413–1 to 413–8, Foxboro Co. Smith, C. A., and Corripio, A. B., Principles and Practice of Automatic

Process Control, New York: John Wiley & Sons, 1997.Stenerson, J., Industrial Automation and Processs Control, Upper Saddle

River, NJ: Prentice Hall, 2002.Stephanopoulos, G., Chemical Process Control: An Introduction to Theory

and Practice, Englewood Cliffs, NJ: Prentice Hall Inc., 1984.


Documents

Transmitters and Local Controllers 3twanclik.free.fr/electricity/IEPOPDF/1081ch3_1.pdf457 Transmitters and Local Controllers 3 3.1 CONTROLLERS—PNEUMATIC 460 History and Development